The field of the invention is low dielectric constant materials.
As design rules in integrated circuits decrease, there is a continuing need for insulator materials with a reduced dielectric constant (k-value) and various approaches have been developed to reduce the dielectric constant of insulator materials closer to the theoretical limit of 1.0 (i.e., the k-value of air).
In one approach, a thermostable polymer is blended with a thermolabile (thermally decomposable) polymer. The blended mixture is then crosslinked and the thermolabile portion thermolyzed to generate nano-sized voids. Examples are set forth in U.S. Pat. No. 5,776,990 to Hedrick et al. Alternatively, thermolabile blocks and thermostable blocks alternate in a single block copolymer, and the block copolymer is then heated to thermolyze the thermolabile blocks. In a further variant, thermostable blocks and thermostable blocks carrying thermolabile portions are mixed and polymerized to yield a copolymer. The copolymer is subsequently heated to thermolyze the thermolabile blocks. Although the process of generating nano-sized voids by thermal destruction is relatively simple, the inclusion of thermolabile portions in a polymer blend or copolymer often has significant limitations. For example, where a blend of polymers is employed, the distribution of the thermolabile portion in the polymer blend is relatively difficult to control. Furthermore, the size of the nano-sized voids tends to be inhomogeneous. Alternatively, while copolymers or block polymers with already built-in thermolabile portions typically allow a better control over pore size and distribution, the synthesis of such co-, and block polymers is often more demanding. Moreover, once the copolymers or block polymers are synthesized, variations in size and quantity of the nano-sized pores are more difficult to achieve.
In another approach, small hollow glass spheres acting as minute void-carriers are blended with a dielectric material, and the dielectric material is subsequently cured in the presence of the void carriers. Examples for the inclusion of small glass spheres are given in U.S. Pat. No. 5,458,709 to Kamezaki and U.S. Pat. No. 5,593,526 to Yokouchi. The inclusion of small glass spheres typically does not result in a reduction of mechanical stability, however, a homogeneous distribution of the glass spheres is often problematic. Moreover, Kamezaki""s and Yokouchi""s approach is generally limited to relatively large glass spheres. To include voids significantly smaller than glass spheres, Rostoker and Pasch describe in U.S. Pat. No. 5,744,399 the use of fullerenes as void-carriers that are blended with a polymeric matrix material which is subsequently crosslinked and cured. In an optional further step, the fullerenes may then be removed by dissolving the fullerenes in a solvent that does not dissolve the matrix material. Rostoker and Pasch""s approach advantageously allows to include significantly smaller void carriers than glass spheres, however, residual solvent may pose potential problems in down-stream processes, especially where multiple layers of low dielectric constant materials are required. Furthermore, size variation of the nano-sized voids is limited due to the limited variation in size of the fullerenes.
Regardless of the approach used to introduce the voids, structural problems are frequently encountered in fabricating nanoporous materials. Among other things, increasing the porosity beyond a critical extent (generally about 30% in the known nanoporous materials) tends to cause many porous materials to collapse. Collapse can be prevented to some degree by adding crosslinking additives that couple thermostable portions with other thermostable portions, thereby producing a more rigid network, and in general two different techniques are known in the art to crosslink thermostable portions in nanoporous materials.
In one technique, specific crosslinking functionalities are already incorporated into the polymer. Such functionalities react together to crosslink polymeric strands prior to thermolysis of a thermolabile portion or removal of other entity to generate nanoporosity. For example. in U.S. Pat. No. 5,177,176 to Auman, polymeric strands are crosslinked using various crosslinking functionalities that are positioned at the end of the polymeric strands. In the other technique, exogenous cross-linking molecules are employed in crosslinking. The crosslinked polymer is then heated to thermolyze the thermolabile portion. In U.S. Pat. No. 5,710,187 to Streckle, Jr., for example, aromatic monomers are crosslinked using exogenously added multifunctional acyl- or benzylic halides.
Although crosslinking with intrinsic or exogenously added crosslinkers often improve structural properties of nanoporous materials at least to some extent, new problems frequently arise from the use of intrinsic and exogenously added crosslinkers. For example, copolymer or block polymers with thermolabile and thermostable portions that include intrinsic crosslinkers are generally difficult to synthesize. Moreover, the choice of suitable intrinsic crosslinkers tends to be further limited by the reactivity and availability of reactive groups in the thermostable portion. On the other hand, where crosslinkers are exogenously added, the solubility, chemical compatibility and selective reactivity of the exogenous crosslinkers are often limiting.
Thus, many approaches are known in the art to reduce the dielectric constant of a polymeric material, however, all or almost all of them suffer from one or more disadvantages. Therefore, there is still a need to provide improved methods and compositions to provide a low dielectric constant material.
The present invention is directed to a low dielectric constant material that comprises a polymeric network, which is fabricated from two components. The first component comprises a polymeric strand, and the second component comprises a star shaped molecule with a central portion from which three or more arms extend. Each of the arms includes a backbone that has a reactive group. The polymeric network is formed upon thermal activation of the first and second component in a reaction that involves at least one of the reactive groups.
In one aspect of the inventive subject matter, the first component comprises a polymeric strand, preferably a poly(arylene), and more preferably a poly(arylene ether), a poly(arylene ether-ether-ketone), a poly(arylene ether-quinoxaline), a poly(arylene ether-benzil), and a poly(arylene ether-quinoline). Further contemplated polymeric strands include polyimides, polyamides, and polyimide-amides.
In another aspect of the inventive subject matter, the second compound comprises a star shaped molecule in which the central portion preferably comprises an adamantane, a diamantane or a fullerene. Other preferred central portions include a silicon atom and at least one aromatic ring.
In a further aspect of the inventive subject matter, at least one of the arms of the star shaped molecule comprises an aromatic ring, and preferably further comprises an ethynyl group, and particularly preferred arms include a 4-ethynylphenyl, a tolanyl, a 4-phenylethynylbiphenyl, and a bistolanyl. It is generally preferred that the reactive group in contemplated arms is a triple bond, and it is further contemplated that the formation of the network (preferably a semi-interpenetrating network) involves a cyclo-addition reaction.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawing.